Hello
I did a bit more research on modes of battery failure and none of them looked that likely.
I managed to recharge the battery this time, and it seemed to charge to the proper voltage and stay there.
I then started doing some tests connecting it to the car. The first couple of time I connected and disconnected there was a current drain of 0.5 to 0.7 amps.
when I had an assistant ready to start pulling fuses, the drain dropped to 0.03 amps and has stayed there ever since. The battery has been in the car for a couple of days now, and all seems to be ok. The voltage drops to 12.6 volts overnight which seems perfectly acceptable.
If I can’t find a fault, I can’t fix the problem! I will be keeping a daily check on voltage to see if I can catch it when a problem occurs.